The metering device is of the type which comprises a housing having a metering opening, whose opening and closing is controlled by the movement of an axially moveable valve needle, an axially extendable piezoelectric actuator cooperating with the valve needle to control its axial movement, a thermal compensator unit cooperating with the piezoelectric actuator and the housing to compensate for different thermal expansion of the housing and the piezoelectric actuator to ensure elastic contact between an end stop of the housing, the piezoelectric actuator and the valve needle, and an electrical connector for supplying electrical power to the piezoelectric actuator.
In such metering devices the housing and the piezoelectric actuator are generally fabricated from different materials and have different thermal coefficients of expansion. Therefore, special measures must be taken to ensure that the injector valve meets the requirements on the fuel flow rate and the geometry of the jet. Particularly important is the influence of the temperature on the principal functional parameters of the injector. The flow rate and other characteristic parameters must remain within predetermined limits of tolerance throughout the full range of the operating temperatures ranging from −40° C. to +150° C.
Specifically, as the piezoelectric actuator generally has a lower coefficient of thermal expansion than the outer housing, it would not maintain Hertzian contact between its fixed end stop surface and the top end of the valve needle. To deal with this problem, the injector valve is typically equipped with a hydraulic thermal compensation unit. As the operation temperature increases, the thermal compensation unit recovers the clearance that would otherwise be created between the valve needle and the piezoelectric actuator.
Due to this fact, the electrical wiring, which connects the upper side of the piezoelectric actuator with the outer side of the injector body, must likewise permit the axial movements, i.e. the extensions and the contractions of the thermal compensator subgroup with high frequency while still providing a reliable electrical connection to the piezoelectric actuator. In current designs, a bipolar and flexible wire coming out of the injector body provides the electrical connection to the piezoelectric actuator. Such a solution, however, can only be employed for test specimens and is not feasible for the standard production of injectors.
In view of the foregoing, it is an object of the present invention to provide a metering device of the above mentioned type with an improved electrical connector which allows for rapid axial movements of the thermal compensator.